1. Field of the Invention
This invention relates to an electrochemical method and apparatus for the synthesis of ammonia. In particular, the invention relates to an anodic electrochemical method and apparatus for the electrosynthesis of ammonia.
2. Background to the Related Art
Ammonia (NH3) is a colorless alkaline gas that is lighter than air and possesses a unique, penetrating odor. Since nitrogen is an essential element to plant growth, the value of nitrogen compounds as an ingredient of mineral fertilizers, was recognized as early as 1840. Until the early 1900's, the nitrogen source in farm soils was entirely derived from natural sources. Haber and Bosch pioneered the synthesis of ammonia directly from hydrogen gas and nitrogen gas on a commercial scale in 1913. Further developments in large-scale ammonia production for fertilizers have made a significant impact on increasing the world's food supply.
Virtually every nitrogen atom of a nitrogen compound travels from the atmosphere to its destined chemical combination by way of ammonia. Industrial uses of ammonia as a nitrogen source have recently consumed a greater share of the total ammonia production, accounting for 20% of the world output. Up to 80% of the ammonia produced is used for the production of nitrogen-based fertilizers, accounting for about 3% of the world's energy consumption. In many developing countries, the capability for ammonia synthesis is the first sign of budding industrialization. In the United States last year there was over 19 billion tons of ammonia produced.
Many methods of ammonia synthesis have been investigated. These methods include the catalytic synthesis of ammonia from its elements using high pressures and high temperatures, indirect ammonia synthesis using steam initiated decomposition of nitrogen based compounds, and the formation of ammonia with the aid of an electrical discharge.
The gas-phase catalytic synthesis of ammonia from its constituent elements, nitrogen gas and hydrogen gas, utilizing an iron-based catalyst at high pressures and high temperatures, is the standard industrial process by which ammonia is produced on an industrial scale worldwide.
                                                        N                              2                ⁢                                  (                  g                  )                                                      +                          3              ⁢                              H                                  2                  ⁢                                      (                    g                    )                                                                                ⁢                                    ⇌                              Iron                -                                  based                  ⁢                                                                          ⁢                  catalyst                                                                                    15                -                                  30                  ⁢                  MPa                                            ;                              430                -                                  480                  ⁢                  °C                                                              ⁢                      2            ⁢                          NH                              3                ⁢                                  (                  g                  )                                                      ⁢                                                  ⁢                          H              298                                      =                              -            45.72                    ⁢                                          ⁢          kJ          ⁢                      /                    ⁢          mol                                    (        1        )            
Since during this gas-phase reaction there is a significant decrease in gas volume as ammonia product is formed, very high pressures must be used to drive the ammonia synthesis reaction to the right of Equation 1, that is in the direction of formation of ammonia gas. The gas-phase synthesis process is an equilibrium process. Thus, carrying out ammonia synthesis at very high pressures is also necessary to prevent back decomposition of synthesized ammonia at the temperatures required to activate the forward reaction process and to provide practical reaction rates. Even then, the equilibrium conversion of hydrogen gas and nitrogen gas to ammonia gas is only on the order of 10 to 15%. Low conversion efficiencies give rise to cost intensive, large scale chemical plants and to costly operating conditions (compression of reactant gases) in order to produce commercially viable hundreds-to-thousands of tons-per-day of ammonia in an ammonia synthesis plant.
Only recently has the feasibility of using electrochemical processes for ammonia synthesis been demonstrated. Except for one, all of the electrochemical processes for the synthesis of ammonia reported to date have involved the cathodic reduction of nitrogen gas at the cathode of an electrochemical cell. Both aqueous-based and organic solvent-based electrolyte solutions have been used at ambient temperature and atmospheric pressure. In these liquid electrolyte solution-based investigations the source of hydrogen (usually in the form of protons) required for the formation of ammonia is provided by the electrochemical decomposition of water or an organic solvent, such as ethanol, at the anodes of the electrochemical cells.
Tsuneto et al., Chemistry Letters, pp. 851-854, 1993, disclosed the use of an ambient temperature electrochemical process utilizing an organic solvent-based electrolyte solution that contained lithium perchlorate as the electrolyte where ammonia gas was formed with a current efficiency of 8% on flowing nitrogen gas at atmospheric pressure over either a titanium metal or silver metal cathode. On using a copper metal cathode and an electrochemical cell temperature of 50° C., a current efficiency of 48% for the production of ammonia was obtained on flowing nitrogen gas at a pressure of 50 atmospheres over the cathode.
Recently, Marnellos and Stoukides published an article entitled “Ammonia Synthesis at Atmospheric Pressure,” Science, vol. 282, Oct. 2, 1998, that disclosed a cathodic electrochemical process for the synthesis of ammonia that avoids the use of aqueous-based or organic solvent-based electrolyte solutions. With this process, electrosynthesis of ammonia takes place at the surface of a porous metal cathode attached to one side of a strontia-ceria-ytterbia (SCY) peroskite solid state proton (H+) conductor. The electrochemical process is operated at atmospheric pressure and 570° C., which is a similar temperature to that used in the Haber-Bosch catalytic process. The apparatus consists of a non-porous, strontia-ceria-ytterbia (SCY) perovskite ceramic tube closed at one end and then further enclosed in a quartz ceramic tube. Electrodes, made from porous polycrystalline palladium films, are deposited on the inner and outer walls of the SCY tube.
Initially, ammonia gas is passed through the system, where the amount of thermal decomposition due to the high operating temperature (570° C.) can be measured. Subsequently, gaseous hydrogen is passed through the quartz tube and over the anode surface, where the hydrogen is converted to protons:3H2→6H++6e−  (2)
The protons are then transported through the proton conducting solid perovskite electrolyte to the cathode surface, on applying an electrical potential between the cathode and the anode, where they come in contact with the nitrogen gas and the following reaction takes place:N2+6H++6e−→2NH3  (3)
Operating at a cell temperature of 570° C. and at atmospheric pressure, greater than 78% of the electrochemically supplied hydrogen from the anode which was transported through the solid electrolyte to the cathode was converted into ammonia. However, the process is limited by slow electrochemical reaction rates due to low proton (H+) fluxes through the solid electrolyte at 570° C. Increasing the temperature to obtain higher proton (H+) fluxes would also increase the rate of thermal decomposition of ammonia.
A major drawback of both low (and high) temperature cathodic electrochemical processes is that the competing hydrogen gas evolution reaction takes place more readily than the formation of ammonia since recombination of adsorbed hydrogen atoms with each other is more likely to occur than reaction between adsorbed hydrogen atoms and adsorbed nitrogen molecules due to the high bond strength (˜1000 kJ mol−1 at 25° C.) of the N≡N triple bond of a nitrogen molecule.
More recently an anodic electrochemical process for the synthesis of ammonia was disclosed in U.S. Pat. No. 6,712,950 which is commonly owned by the assignee of the present application. This new anodic electrochemical process overcomes many of the limitations of the earlier discussed cathodic electrochemical processes. The anodic process uses molten salts selected from those having melting points that range from room temperature to greater than 400° C. and containing a dissolved nitride ion-containing salt, such as lithium nitride (Li3N), as the electrolyte. The anode is comprised of either a porous structure or a membrane permeable to hydrogen gas. Hydrogen is introduced into the electrochemical cell at the anode/molten salt electrolyte interface. The cathode is also comprised of a porous structure and nitrogen gas is introduced into the electrochemical cell at the cathode/molten salt electrolyte interface.
On allowing current to flow through the electrochemical cell, a nitrogen gas molecule is reduced to nitride ions (N3−) at the cathode/molten salt electrolyte interface, as represented by Equation 4:N2+6e−→2N3−  (4)
Due to the applied electrical potential between the cathode and the anode, nitride ions (N3−) migrate from the cathode/molten salt electrolyte interface to the anode/molten salt electrolyte interface. At the anode/molten salt electrolyte interface, nitride ions (N3−) are oxidized to produce adsorbed nitrogen atoms, as represented by Equation 5:2N3−→2Nads+6e−  (5)
Adsorbed nitrogen atoms react with either adsorbed hydrogen molecules, or more likely with adsorbed hydrogen atoms, on the surface of the anode to produce ammonia gas molecules as represented by Equation 6:2Nads+6Hads→2NH3  (6)
With this process a current efficiency of over 50% was obtained for the production of ammonia.
The formation of nitride ions (N3−) at the cathode by the electrochemical reduction of nitrogen gas molecules and their conversion at the anode to give adsorbed nitrogen atoms by electrochemical oxidation of nitride ions (N3−) forms the basis of the anodic process for the production of ammonia. In this anodic process, the nitride anion (N3−) is the only electrochemically active anionic species present in the molten salt that participates in the formation of ammonia gas. Hydrogen gas molecules, or more preferably adsorbed hydrogen atoms, participate in a subsequent chemical step and it is believed that the current efficiency for the formation of ammonia is controlled by the successful reaction between adsorbed nitrogen atoms and adsorbed hydrogen atoms on the surface of the anode. Sufficient coverage of the anode surface with adsorbed hydrogen atoms is dependent on the dissociative adsorption of hydrogen gas molecules under the operating conditions of temperature and pressure in a molten salt environment and also by the affinity of the surface of the anode electrocatalyst for adsorbed hydrogen species.
Therefore, there remains a need for an improved method of producing ammonia.
It would be desirable if the improved anodic method could produce ammonia at lower temperatures and lower pressures, while achieving a greater conversion than existing methods. It would be even further desirable if the improved anodic electrochemical method were compatible with existing process units, such as being able to use the same hydrogen and nitrogen sources as are used in the Haber-Bosch process.